The present invention relates to a steering force control apparatus suitable for the purpose of obtaining a required steering force by controlling a hydraulic reaction mechanism in a power steering system in accordance with various vehicle running conditions such as a running speed and a steering angle.
A steering force control apparatus using a hydraulic reaction mechanism is known as a steering force control apparatus for performing steering force control in accordance with various vehicle running conditions such as a running speed and a steering angle in a power steering system for reducing a steering wheel operating force (steering force) of an automobile.
Such a steering force control apparatus using a hydraulic reaction mechanism controls the magnitude of a reaction oil pressure in accordance with various vehicle running conditions and uses this reaction oil pressure as a selective constraining force between input and output shafts of a power steering system, thereby selectively constraining the two shafts or allowing them to rotate relative to each other.
For example, steering control is performed such that when a vehicle is kept stopped or running at low speeds, the reaction oil pressure is minimized to reduce the constraining force to enable a light steering operation; when a vehicle is running at high speeds, the reaction oil pressure is increased to obtain a large constraining force, thereby giving heaviness to a steering wheel to ensure stability in straight driving.
An example of a hydraulic reaction mechanism of this type is disclosed in U.S. Pat. No. 4,787,469. In this prior application, a reaction oil pressure is shunted from a portion of a main oil pressure passage extending from a pump to a power cylinder through a flow path switching valve, and introduced to a hydraulic reaction chamber for moving a reaction plunger, under the control of a reaction oil pressure control valve, such as a spool valve.
In this conventional apparatus, the reaction oil pressure control valve is commonly constituted by an electrical actuator, such as a solenoid coil or a stepping motor, capable of generating a necessary actuating force using an output current from a controller in accordance with detection signals from, e.g., a vehicle speed sensor, a steering angle sensor, and a torque sensor. Such electronic control can properly, reliably operate the hydraulic reaction mechanism to perform steering control according to a vehicle speed or a steering condition.
The above conventional apparatuses for performing steering force control using the hydraulic reaction mechanism, however, have a problem in sealing properties in the hydraulic reaction chamber. In addition, there is another problem derived from throttles and the like present in a hydraulic circuit. Because of these problems, the reaction oil pressure characteristic in the hydraulic reaction chamber sometimes temporarily differs from the characteristic of a pressure supplied from the hydraulic reaction force control valve. This consequently brings about a sense of unnaturalness as a so-called "discontinuity phenomenon" in which a steering torque does not increase smoothly during steering operation, resulting in an inability to perform the required steering force control.
For example, in a steering force control apparatus using a hydraulic reaction force, which is disclosed in, for example, U.S. Pat. No. 4,899,842, a hydraulic reaction chamber is formed around an output shaft in a housing, and a ring-like reaction piston is provided to be slidable in the axial direction at one end of the hydraulic reaction chamber. One end of the hydraulic reaction chamber is closed by seal rings arranged on the inner and outer circumferential surfaces of this piston; this separates the hydraulic reaction chamber from a low-pressure chamber, which is formed adjacent to the hydraulic reaction chamber to constitute a hydraulic reaction mechanism, and in which balls and a reaction force receiving unit are arranged. At the other end of the hydraulic reaction chamber, a ring-like partitioning member is fitted on the output shaft. A seal ring interposed between the outer circumferential surface of this partitioning member and the housing separates the hydraulic reaction chamber from a low-pressure chamber formed at the outer end of the hydraulic reaction chamber, thereby sealing the hydraulic reaction chamber.
The hydraulic reaction chamber is connected to the output port of a hydraulic reaction force control valve. The low-pressure chambers formed adjacent to the hydraulic reaction chamber at its two end portions are connected to a tank through discharge passages formed in a power steering main body and having throttles or the like at arbitrary positions.
In the above steering force control apparatus, the hydraulic reaction force control valve obtains a necessary reaction oil pressure by shunting a pressure oil from a pump to the hydraulic reaction chamber and to a low-pressure chamber on the tank side by means of variable throttles. The low-pressure chamber in this hydraulic reaction force control valve is connected to the tank through a path (e.g., a path indicated by a broken line in FIG. 1 to be described later) independent of the discharge passages from the neighboring low-pressure chambers of the hydraulic reaction chamber, in the power steering main body.
In the above conventional apparatus, however, a pressure Pr in the hydraulic reaction chamber and pressures P1 and P2 in the low-pressure chambers adjacent to the hydraulic reaction chamber sometimes take a relation Pr&lt;P1, P2 under the influences of the respective oil pressure passages or the throttles midway along the passages. From this state, the reaction oil pressure Pr rises with a steering operation to meet a relation Pr&gt;P1, P2.
Such a pressure change causes a change in direction in which the oil pressure acts on the seal rings for sealing the hydraulic reaction chamber from the neighboring low-pressure chambers at the two ends of the hydraulic reaction chamber, during the steering operation, and this degrades the sealing properties obtained in the hydraulic reaction chamber by the sealing rings.
This problem of reaction oil pressure variation in the hydraulic reaction chamber due to the leakage described above is serious particularly in the relationship with the low-pressure chamber which is adjacent to the hydraulic reaction chamber via the reaction piston. Therefore, it is necessary to establish a required relationship in pressure difference between these two chambers in order to perform steering force control using the hydraulic reaction force properly.
The degradation in sealing performance is liable to occur when the pressure relationship between Pr, P1, and P2 is reversed. The consequent leakage of the reaction oil pressure Pr and the pressure rise on the back pressure side, i.e., on the side of the low-pressure chambers at the two ends of the hydraulic reaction chamber cause a reduction in pressure difference; this sometimes makes it impossible to obtain required hydraulic reaction force rise characteristics in the hydraulic reaction chamber. As can be seen from a characteristic curve indicating an input torque-output oil pressure relationship shown in FIG. 4, if the output oil pressure increases due to a slight rise in the input torque, projections indicated by alternate long and two dashed lines take place. This may lead to an inconvenience that a steering wheel is turned too easily. Hence, a demand has arisen for a certain countermeasure capable of solving this problem.